The ship of Theseus, also known as Theseus' paradox, is a thought experiment that raises the question of whether an object which has had all of its components replaced remains fundamentally the same object. The paradox is most notably recorded by Plutarch in Life of Theseus from the late first century. Plutarch asked whether a ship which was restored by replacing each and every one of its wooden parts remained the same ship.

The paradox had been discussed by more ancient philosophers such as Heraclitus, Socrates, and Plato prior to Plutarch's writings; and more recently by Thomas Hobbes and John Locke. Several variants are known, notably "grandfather's axe". This thought experiment is "a model for the philosophers"; some say, "it remained the same," some saying, "it did not remain the same"